Economic pressure is also increasing on Catalan separatist leaders with more major companies moving head offices away from the region.

Infrastructure firm Abertis, property group Inmobiliaria Colonial and telecoms company Cellnex all announced plans to relocate to Madrid.

Analysis: Will Puigdemont make good his threat?

Tom Burridge, BBC News, Barcelona

Never before has the world watched Catalonia's regional parliament so closely. This evening, with probably large numbers of police and protesters outside, the leader of the devolved government is due to address Catalan MPs. Carles Puigdemont plans to present his government's results of the disputed referendum, which Madrid declared illegal.

Since then, Mr Puigdemont has said on several occasions that he will declare independence, despite fierce opposition across Spain and criticism from European governments.

Now, will he make good his threat? Or will he announce a more nuanced strategy, hoping still for a proper, recognised referendum one day? Under pressure to act, the Spanish government has made stark warnings, too. If the order comes, thousands of Spanish national police, here in Barcelona, could intervene.

Publishing company Grupo Planeta said it would also move from Barcelona to Madrid if there was a declaration of independence.

Major lenders Caixabank and Sabadell have already announced they are moving offices out of the region.

Catalonia is one of Spain's wealthiest regions, accounting for a quarter of the country's exports.